1. Field of the Invention
The present invention relates to an optical system, and is suitable for an optical system such as a camera for a silver salt film, a digital still camera, a video camera, a telescope, binoculars, a projector, or a copying machine.
2. Description of the Related Art
In general, in an optical system for use in a digital camera, a video camera or the like, when the total length of a lens is reduced to miniaturize the whole optical system, a large number of aberrations, more particularly chromatic aberrations such as axial chromatic aberration (longitudinal chromatic aberration) and chromatic aberration of magnification (lateral chromatic aberration) are generated, and the optical performance tends to drop. Especially in a telephoto type optical system in which the total lens length is reduced, the longer the focal length is, the more chromatic aberrations are generated. Here, the total lens length refers to the total optical length, and is the length from the front first lens surface to the image plane (or object plane).
As a method of reducing the generation of such chromatic aberration, there is known a method using an extraordinary partial dispersion material in an optical material, or a method using an diffractive optical element in an optical path.
In the telephoto type optical system, it is general to reduce the chromatic aberration using a positive-refractive-power lens constituted of a low-dispersion optical material (optical member having a large Abbe number) such as fluorite having extraordinary partial dispersion, and a negative-refractive-power lens constituted of a high-dispersion optical material in a front lens unit in which the passage position from the optical axis is comparatively high. There have been proposed various optical systems of such telephoto type (see, Japanese Examined Patent Publication (Kokoku) No. 60-49883 (corresponding to U.S. Pat. No. 4,241,983), Japanese Examined Patent Publication (Kokoku) No. 60-55805 (corresponding to U.S. Pat. No. 4,348,084) and Japanese Unexamined Patent Publication (Kokai) No. 11-119092 (corresponding to U.S. Pat. No. 6,115,188)).
Here, the paraxial marginal ray is a paraxial ray for the case when the focal length of the whole optical system is normalized into 1, and a ray having a height 1 from the optical axis is allowed to enter the system in parallel with the optical axis of the optical system. Additionally, it is assumed that an object is disposed on the left side of the optical system, and the light entering the optical system from the object side is treated as the light traveling from the left to the right. The paraxial chief ray is a paraxial ray passing through the intersection of the entrance pupil and the optical axis of the optical system among incident rays at −45° with respect to the optical axis in a case where the focal length of the whole optical system is normalized into 1. The incidence angle on the optical system is measured from the optical axis, a clockwise angle is regarded as positive, and a counterclockwise angle is regarded as negative.
Moreover, there is known a telephoto type optical system in which the chromatic aberration has been corrected using the diffractive optical element without using any abnormal portion dispersed optical material (see, Japanese Unexamined Patent Publication (Kokai) No. 6-324262 (corresponding to U.S. Pat. No. 5,790,321) and Japanese Unexamined Patent Publication (Kokai) No. 6-331887 (corresponding to U.S. Pat. No. 5,629,799)). In Japanese Unexamined Patent Publication (Kokai) No. 6-324262 and Japanese Unexamined Patent Publication (Kokai) No. 6-331887, a telephoto type optical system having an F number of about F 2.8 is disclosed in which the diffractive optical element is combined with a refractive optical element to thereby correct the chromatic aberration comparatively satisfactorily.
In general, the diffractive optical element, an absolute value of the numeric value corresponding to the Abbe number is as small as 3.45. The element has a characteristic that the chromatic aberration can be largely changed while hardly influencing spherical aberration, comatic aberration, and astigmatism, when the power (inverse number of a focal length) by diffraction is only slightly changed. Since diffraction light is to be handled, the power linearly changes with respect to the change of the wavelength of the incident light, and the wavelength characteristic of the chromatic aberration coefficient forms a completely straight line.
Therefore, to reduce the total lens length, corrections of the spherical aberration, the comatic aberration, and the astigmatism may be specified in performing the aberration correction. Moreover, since the chromatic aberration is corrected by the diffractive optical element, the glass material and the refractive power of the material of a constituent lens may be optimized to allow designing in such a manner as to obtain a linearity of the wavelength characteristic of the chromatic aberration coefficient regardless of the absolute amount of the chromatic aberration worsened by the reduction of the total length. As a result, a telephoto type optical system can be obtained in which the total lens length has been reduced.
Moreover, as the optical material having the function of correcting the chromatic aberration similar to the optical characteristic of the diffractive optical element, a liquid material is known which exhibits comparatively highly dispersed and comparatively extraordinary partial dispersion characteristics, and an achromatic optical system using the material is proposed (see, U.S. Pat. No. 5,731,907 and U.S. Pat. No. 5,638,215).
In a telephoto type optical system in which fluorite or the like is used in the optical material as disclosed in Japanese Examined Patent Publication (Kokoku) No. 60-49883, Japanese Examined Patent Publication (Kokoku) No. 60-55805 and Japanese Unexamined Patent Publication (Kokai) No. 11-119092, it is easy to correct the chromatic aberration in a case where the total lens length is made comparatively long. However, when the total lens length is reduced, many chromatic aberrations are generated, and it is difficult to correct them satisfactorily. In this method, the chromatic aberration generated in a front lens system having the positive refractive power is simply reduced utilizing the low dispersion and extraordinary partial dispersion present in a material such as fluorite. Even when worsened chromatic aberration accompanying the reduction of the total lens length is to be corrected, the chromatic aberration does not largely change unless the refractive power of the lens surface is largely changed in the lens using low-dispersion glass such as fluorite having a large Abbe number. Therefore, it is difficult to establish both correction of the chromatic aberration and correction of aberrations such as spherical aberration, comatic aberration, and astigmatism generated by the increasing the refractive power.
On the other hand, although the diffractive optical element has a sufficient function of correcting the chromatic aberration, diffraction light having an unnecessary diffraction order is generated in addition to diffraction light having a designed diffraction order for actual use, and a problem occurs that the unnecessary diffraction light forms colored flare light to spoil the image forming performance. As a method of reducing the unnecessary diffraction light, a method is known in which there is used a so-called laminated type diffractive optical element comprising a plurality of blaze type diffraction gratings laminated in the optical axis direction. Therefore, it is possible to concentrate effort on the designed diffraction order, and reduce the unnecessary diffraction light largely. However, there is a problem that flare by the unnecessary diffraction light still appears when a high-luminance object is photographed.
Moreover, as a method of manufacturing the diffractive optical element, a method is known in which the element is molded of an ultraviolet cured resin or the like. However, in this method, since sensitivity of diffraction efficiency of the diffractive optical element is remarkably high during manufacture, a very high mold or molding precision is required. As a result, it is difficult to manufacture the element, and manufacturing costs rise.
Since the materials disclosed in U.S. Pat. No. 5,731,907 and U.S. Pat. No. 5,638,215 are liquids, a structure to introduce and seal the liquid is required. In a case where the liquid is used in the optical material, it is difficult to manufacture the element. Characteristics such as refractive index and dispersion largely change with a temperature change, resistance to environment is not sufficient. Furthermore, since an interface between the liquid and air is not obtained, the sufficient ability to correct chromatic aberration is not easily obtained.